Light emitting diodes and laser diodes are well known solid state electronic light emitting devices capable of generating light upon application of a sufficient voltage. Light emitting diodes and laser diodes may be generally referred to as light emitting devices (“LEDs”). Light emitting devices generally include a p-n junction formed in an epitaxial layer grown on a substrate such as sapphire, silicon, silicon carbide, gallium arsenide and the like. The wavelength distribution of the light generated by the LED generally depends on the material from which the p-n junction is fabricated and the structure of the thin epitaxial layers that make up the active region of the device.
Typically, an LED chip includes a substrate, an n-type epitaxial region formed on the substrate and a p-type epitaxial region formed on the n-type epitaxial region (or vice-versa). To facilitate application of a voltage to the device, an anode ohmic contact is formed on a p-type region of the device (typically, an exposed p-type epitaxial layer) and a cathode ohmic contact is formed on an n-type region of the device (such as the substrate or an exposed n-type epitaxial layer).
To use an LED chip in a circuit, it is known to enclose an LED chip in a package to provide environmental and/or mechanical protection, color selection, focusing and the like. An LED package also includes electrical leads, contacts or traces for electrically connecting the LED package to an external circuit. In a typical LED package, an LED chip is mounted on a reflective cup by means of a solder bond or conductive epoxy. One or more wirebonds connect the ohmic contacts of the LED chip to leads which may be attached to or integral with the reflective cup. The reflective cup may be filled with an encapsulant material containing a wavelength conversion material such as a phosphor. Light emitted by the LED at a first wavelength may be absorbed by the phosphor, which may responsively emit light at a second wavelength. The entire assembly is then encapsulated in a clear protective resin, which may be molded in the shape of a lens to collimate the light emitted from the LED chip. Some LED packages have one or more LED chips mounted onto a carrier such as a printed circuit board (PCB) carrier.
During operation of the LED or LEDs, large amounts of heat may be generated. Much of the heat may be dissipated by the substrate and the reflector cup, each of which may act as a heatsink for the package. However, the temperature of the package may still increase significantly during operation. Substrates typically have low thermal conductivity. Additionally, while the reflective cup directs light upward, some light may be absorbed by the reflector cup or there may be reasons not to use a separate metal piece as a metal reflector.
Encapsulant materials, such as silicone gels, typically have high coefficients of thermal expansion. As a result, when the package heats up, the encapsulant material may expand. As the lens is mounted within a channel defined by the sidewalls of the reflector cup, the lens may travel up and down within the sidewalls as the encapsulant material expands and contracts. Expansion of the encapsulant material may extrude the encapsulant into spaces or out of the cavity such that, when cooled, it may not move back into the cavity. This could cause delamination, voids, higher triaxial stresses and/or the like, which may result in less robust light emitting devices. Accordingly, there continues to exist a need in the art for more effective methods for heat spreading and lowering thermal resistances of LED systems.